The overall goal of this research is to develop stimulation strategies that increase the number of effective channels for cochlear implant (CI) users. Most CI users have great difficulty in challenging listening conditions (e.g., background noise, competing speech, music, etc.), due to the limited spectral resolution of the implant device. While contemporary CI devices can transmit as many as 22 physical channels (and many more virtual channels), CI users can typically access only ~8 channels. We hypothesize that CI users'functional spectral resolution is primarily limited by channel interactions, i.e., the electrical current spread between channels. One approach to reduce current spread is to utilize tripolar and/or quadrupolar stimulation modes (current focusing). However, even 22 fully independent physical channels are not sufficient to support challenging listening tasks. One approach to increase the number of channels beyond the physical number of electrodes is to simultaneously stimulate adjacent electrodes, creating virtual channels (current steering). We propose to combine current focusing with current steering to improve CI users'functional spectral resolution. If successful, combining current focusing and current steering within a signal processing strategy may allow CI users to access the additional spectral channels needed for difficult listening conditions. In order to successfully implement such a strategy, it is important to develop the basic psychophysical framework of current focusing and current steering. The following three specific aims are to collect the basic information needed to develop the next generation sound processing strategy. In Specific Aim 1, we will compare the current spread between loudness balanced stimuli with differing degrees of current focusing. We predict that for a fixed loudness level, current spread will be significantly reduced for focused stimuli. If so, focused stimulation in a signal processing strategy should reduce channel interaction and improve the functional spectral resolution. In Specific Aim 2, we will investigate perceptual effects associated with different degrees of current focusing. Anecdotal reports suggest that current focusing may result in changes in pitch (Saoji, 2007;Berenstein, 2007) and/or tonal clarity (Marzalek et al., 2007). We predict that increased focusing will provide a small but reliable improvement in tonal clarity, but will not systematically alter the perceived pitch. In Specific Aim 3, we will measure discrimination of current-steered virtual channels that can be generated between two electrodes with and without current focusing. We predict that spatial selectivity for virtual channels will be significantly improved with current focusing. Relevance: While contemporary cochlear implants (CIs) can transmit as many as 22 physical channels (and many more virtual channels), hearing impaired patients with CIs can typically access only ~8 channels. Challenging listening conditions (e.g., speech in noise, competing speech, music, etc) require significantly more channels. The proposed research is clinically relevant, as it addresses a long-standing challenge in CI research: how to increase the number of effective spectral channels.